Impact of extreme and fluctuating temperatures on aphid-parasitoid dynamics

Effects of Constant versus Fluctuating Temperatures on Fitness Indicators of the Aphid Dysaphis plantaginea and the Parasitoid Aphidius matricariae

Simple Summary

Like all organisms, insects encounter temperatures that fluctuate on different time scales: within a day, between days, or throughout the seasons. However, most studies on the impact of temperature on insect physiology, behavior, morphology, or ecology have focused on constant temperatures tested in the laboratory. In our study, we wanted to know if fluctuating temperatures during the day (7–17 °C, average 12 °C) can affect insects differently compared to a constant temperature of 12 °C. We used, as a model, the apple aphid Dysaphis plantaginea, a major threat to apple orchards worldwide, and its parasitoid Aphidius matricariae, which is used in biological control. We found that many traits—but not all—were affected. In particular, the fluctuating thermal regime reduced the development time of aphids and parasitoids, improved the rate of parasitism, and tended (albeit slightly) to increase the longevity of both species. In contrast, we did not find strong effects on morphological traits. Our results can be used to better predict how these agronomically important insects behave in orchards, how ecologically-relevant fluctuating temperatures affect host–parasitoid relationships, and ultimately what the implications are in the context of climate change and biological control.

Abstract

Testing fluctuating rather than constant temperatures is likely to produce more realistic datasets, as they are ecologically more similar to what arthropods experience in nature. In this study, we evaluated the impact of three constant thermal regimes (7, 12, and 17 °C) and one fluctuating thermal regime (7–17 °C with a mean of 12 °C) on fitness indicators in the rosy apple aphid Dysaphis plantaginea, a major pest of apple orchards, and the parasitoid Aphidius matricariae, one of its natural enemies used in mass release biological control strategies. For some—but not all—traits, the fluctuating 7–17 °C regime was beneficial to insects compared to the constant 12 °C regime. Both aphid and parasitoid development times were shortened under the fluctuating regime, and there was a clear trend towards an increased longevity under the fluctuating regime. The fecundity, mass, and size were affected by the mean temperature, but only the mass of aphids was higher at 7–17 °C than at a constant 12 °C. Parasitism rates, but not emergence rates, were higher under the fluctuating regime than under the constant 12 °C regime. Results are discussed within the framework of insect thermal ecology and Jensen’s inequality. We conclude that incorporating thermal fluctuations in ecological studies could allow for the more accurate consideration of how temperature affects host–parasitoid interactions and insect responses to temperature change over time.

Dealing with predictable and unpredictable temperatures in a climate change context: the case of parasitoids and their hosts

The Earth's climate is changing at a rapid pace. To survive in increasingly fluctuating and unpredictable environments, species can either migrate or evolve through rapid local adaptation, plasticity and/or bet-hedging. For small ectotherm insects, like parasitoids and their hosts, phenotypic plasticity and bet-hedging could be critical strategies for population and species persistence in response to immediate, intense and unpredictable temperature changes. Here, we focus on studies evaluating phenotypic responses to variable predictable thermal conditions (for which phenotypic plasticity is favoured) and unpredictable thermal environments (for which bet-hedging is favoured), both within and between host and parasitoid generations. We then address the effects of fluctuating temperatures on host-parasitoid interactions, potential cascading effects on the food web, as well as biological control services. We conclude our review by proposing a road map for designing experiments to assess if plasticity and bet-hedging can be adaptive strategies, and to disentangle how fluctuating temperatures can affect the evolution of these two strategies in parasitoids and their hosts.

Multigenerational experimental simulation of climate change on an economically important insect pest

Long-term multigenerational experimental simulations of climate change on insect pests of economically and socially important crops are crucial to anticipate challenges for feeding humanity in the not-so-far future. Mexican bean weevil Zabrotes subfasciatus, is a worldwide pest that attacks the common bean Phaseolus vulgaris seeds, in crops and storage. We designed a long term (i.e., over 10 generations), experimental simulation of climate change by increasing temperature and CO2 air concentration in controlled conditions according to model predictions for 2100. Higher temperature and CO2 concentrations favored pest's egg-to-adult development survival, even at high female fecundity. It also induced a reduction of fat storage and increase of protein content but did not alter body size. After 10 generations of simulation, genetic adaptation was detected for total lipid content only, however, other traits showed signs of such process. Future experimental designs and methods similar to ours, are key for studying long-term effects of climate change through multigenerational experimental designs.

Ecologically relevant thermal fluctuations enhance offspring fitness: biological and methodological implications for studies of thermal developmental plasticity

Natural thermal environments are notably complex and challenging to mimic in controlled studies. Consequently, our understanding of the ecological relevance and underlying mechanisms of organismal responses to thermal environments is often limited. For example, studies of thermal developmental plasticity have provided key insights into the ecological consequences of temperature variation, but most laboratory studies use treatments that do not reflect natural thermal regimes. While controlling other important factors, we compared the effects of naturally fluctuating temperatures with those of commonly used laboratory regimes on development of lizard embryos and offspring phenotypes and survival. We incubated eggs in four treatments: three that followed procedures commonly used in the literature, and one that precisely mimicked naturally fluctuating nest temperatures. To explore context-dependent effects, we replicated these treatments across two seasonal regimes: relatively cool temperatures from nests constructed early in the season and warm temperatures from late-season nests. We show that natural thermal fluctuations have a relatively small effect on developmental variables but enhance hatchling performance and survival at cooler temperatures. Thus, natural thermal fluctuations are important for successful development and simpler approximations (e.g. repeated sine waves, constant temperatures) may poorly reflect natural systems under some conditions. Thus, the benefits of precisely replicating real-world temperatures in controlled studies may outweigh logistical costs. Although patterns might vary according to study system and research goals, our methodological approach demonstrates the importance of incorporating natural variation into controlled studies and provides biologists interested in thermal ecology with a framework for validating the effectiveness of commonly used methods.

The Fitness and Economic Benefits of Rearing the Parasitoid Telenomus podisi Under Fluctuating Temperature Regime

Successful biological control requires detailed knowledge about the mass rearing conditions of the control agents in order to ensure higher quality of field-released insects. Thus, we investigated whether rearing fluctuating thermal condition would affect the fitness and costs of the parasitoid wasp Telenomus podisi Ashmead (a biocontrol agent used for controlling the Neotropical brown stink bug Euschistus heros (Fabricius)) when compared with parasitoid reared at constant temperature condition, which is commonly used in insect facilities. Parasitoids were reared under either constant (continuous exposure at 25 ± 2°C) or fluctuating temperature conditions (i.e., 30 ± 2°C during day and 20 ± 2°C at night) during four consecutive generations. Our results indicated that tested fluctuating temperature is more suitable for rearing of T. podisi as such temperature condition not only resulted in fitness benefits (e.g., shorter developmental time, longer female longevity, higher fecundity/fertility) but also reduced (approximately 23.5%) the estimated costs for producing the parasitoids. Furthermore, rearing T. podisi under fluctuating temperatures improved tolerance to low constant temperatures (i.e., 20°C) without changing the tolerance to constant high temperatures (30°C) in the fourth generation. Surprisingly, even parasitoids that developed under fluctuating thermal conditions performed better than those reared at constant temperature of 25°C. Collectively, our findings suggest that T. podisi reared under fluctuating thermal condition can tolerate better fluctuating temperatures that normally occur both during long periods of transport and in agricultural ecosystems, which will increase the quality and productivity of mass-reared T. podisi for inundative releases.

Shifts in temperature influence how Batrachochytrium dendrobatidis infects amphibian larvae

Many climate change models predict increases in frequency and magnitude of temperature fluctuations that might impact how ectotherms are affected by disease. Shifts in temperature might especially affect amphibians, a group with populations that have been challenged by several pathogens. Because amphibian hosts invest more in immunity at warmer than cooler temperatures and parasites might acclimate to temperature shifts faster than hosts (creating lags in optimal host immunity), researchers have hypothesized that a temperature shift from cold-to-warm might result in increased amphibian sensitivity to pathogens, whereas a shift from warm-to-cold might result in decreased sensitivity. Support for components of this climate-variability based hypothesis have been provided by prior studies of the fungus Batrachochytrium dendrobatidis (Bd) that causes the disease chytridiomycosis in amphibians. We experimentally tested whether temperature shifts before exposure to Batrachochytrium dendrobatidis (Bd) alters susceptibility to the disease chytridiomycosis in the larval stage of two amphibian species–western toads (Anaxyrus boreas) and northern red legged frogs (Rana aurora). Both host species harbored elevated Bd infection intensities under constant cold (15° C) temperature in comparison to constant warm (20° C) temperature. Additionally, both species experienced an increase in Bd infection abundance after shifted from 15° C to 20° C, compared to a constant 20° C but they experienced a decrease in Bd after shifted from 20° C to 15° C, compared to a constant 15° C. These results are in contrast to prior studies of adult amphibians highlighting the potential for species and stage differences in the temperature-dependence of chytridiomycosis.

Sporadic short temperature events cannot be neglected in predicting impacts of climate change on small insects

Climate warming is characterized by increase in extreme heat events (EHEs). EHEs and mild temperature periods alternate with each other and form complex climate scenarios. Among these scenarios, low-frequency and short-duration extreme heat events during long mild periods (sporadic short EHEs) and low-frequency and short-duration mild periods during long extreme heat events (sporadic short mild periods) commonly occur in nature. The biological effects of these two types of temperature events have not been thoroughly elucidated to date. To clarify the biological effects of these temperature events on organisms, we selected the English grain aphid, a globally important cereal pest, as our model system. We exposed aphids to simulated 24-h diurnal fluctuating temperatures, inserted these events during the wheat growing season and then investigated development, adult longevity, fecundity, survival, and demographic parameters. We found that sporadic short mild periods during a long EHE could improve their life history traits. Increasing the duration of mild periods from 1 day to 2 days did not significantly change their demographic performance. Sporadic short EHEs during a long mild period did not significantly affect vital rates, while increasing the duration of EHEs from 1 day to 2 days worsened the aphids' performance. We found that short mild episodes in the hot season may benefit small insects to buffer long duration heatwaves. We discussed how sporadic short mild periods during a long EHE supplied aphids a chance to recover from heat stress. Thus, we suggest that sporadic temperature events should be considered in population prediction of small insects under climate change and should be integrated into pest management.

Upper thermal limits differ among and within component species in a tritrophic host-parasitoid-hyperparasitoid system

Understanding how climate change affects host-parasite systems and predicting the consequences for ecosystems, economies, and human health has emerged as an important task for science and society. Some basic insight into this complex problem can be gained by comparing the thermal physiology of interacting host and parasite species. In this study, we compared upper thermal tolerance among three component species in a natural host-parasitoid-hyperparasitoid system from Virginia, USA. To assess the ecological relevance of our results, we also examined a record of maximum daily air temperatures collected near the study site in the last 124 years. We found that the caterpillar host Manduca sexta had a critical thermal maximum (CTmax) about 4°C higher than the parasitic wasp, Cotesia congregata, and the hyperparasitic wasp, Conura sp., had a CTmax about 6°C higher than its host, C. congregata. We also found significant differences in CTmax among instars and between parasitized and non-parasitized M. sexta. The highest maximum daily air temperature recorded near the study in the last 124 years was 42°C, which equals the average CTmax of one species (C. congregata) but is several degrees lower than the average CTmax of the other two species (M. sexta, Conura sp.) in this study. Our results combined with other studies suggest that significant differences in thermal performance within and among interacting host and parasite species are common in nature and that climate change may be largely disruptive to these systems with responses that are highly variable and complex.

Warming Accelerates Carbohydrate Consumption in the Diapausing Overwintering Peach Fruit Moth Carposina sasakii (Lepidoptera: Carposinidae)

Climate warming provides a challenge for small insects persisting in cold seasons through diapause because they fail to accumulate and maintain adequate reserves to complete this stage successfully. One way of understanding this challenge is to follow physiological changes in these insects under higher temperatures, including the consumption and allocation of energy reserves during and after diapause. We simulated autumn and spring warming conditions to study carbohydrate consumption dynamics during diapause-post-diapause periods by monitoring shifts in carbohydrate levels in a facultative diapause species, the peach fruit moth Carposina sasakii Matsumura (Lepidoptera: Carposinidae). We found carbohydrates were rapidly consumed in the post-diapause phase, which might lead to a trade-off in the allocation of energy reserves between diapause maintenance and post-diapause development. This suggests that temperature increases in autumn and spring may alter diapause maintenance and post-diapause development through changing carbohydrate levels.

Effects of daily fluctuating temperatures on the Drosophila-Leptopilina boulardi parasitoid association

Koinobiont parasitoid insects, which maintain intimate and long-term relationships with their arthropod hosts, constitute an association of ectothermic organisms that is particularly sensitive to temperature variations. Because temperature shows pronounced natural daily fluctuations, we examined if experiments based on a constant temperature range can mask the real effects of the thermal regime on host-parasitoid interactions. The effects of two fluctuating thermal regimes on several developmental parameters of the Drosophila larval parasitoid Leptopilina boulardi were analyzed in this study. Regime 1 included a range of 16-23-16°C and regime 2 included a range of 16-21-26-21-16°C (mean temperature 20.1°C) compared to a 20.1°C constant temperature. Under an average temperature of 20.1°C, which corresponds to a cold condition of L. boulardi development, we showed that the success of parasitism is significantly higher under a fluctuating temperature regime than at constant temperature. A fluctuating regime also correlated with a reduced development time of the parasitoids. In contrast, the thermal regime did not affect the ability of Drosophila to resist parasitoid infestation. Finally, we demonstrated that daily temperature fluctuation prevented the entry into diapause for this species, which is normally observed at a constant temperature of 21°C. Overall, the results reveal that constant temperature experiments can produce misleading results, highlighting the need to study the thermal biology of organisms under fluctuating regimes that reflect natural conditions as closely as possible. This is particularly a major issue in host-parasitoid associations, which constitute a good model to understand the effect of climate warming on interacting species.

Future Scenarios for Plant Virus Pathogens as Climate Change Progresses

Knowledge of how climate change is likely to influence future virus disease epidemics in cultivated plants and natural vegetation is of great importance to both global food security and natural ecosystems. However, obtaining such knowledge is hampered by the complex effects of climate alterations on the behavior of diverse types of vectors and the ease by which previously unknown viruses can emerge. A review written in 2011 provided a comprehensive analysis of available data on the effects of climate change on virus disease epidemics worldwide. This review summarizes its findings and those of two earlier climate change reviews and focuses on describing research published on the subject since 2011. It describes the likely effects of the full range of direct and indirect climate change parameters on hosts, viruses and vectors, virus control prospects, and the many information gaps and deficiencies. Recently, there has been encouraging progress in understanding the likely effects of some climate change parameters, especially over the effects of elevated CO2, temperature, and rainfall-related parameters, upon a small number of important plant viruses and several key insect vectors, especially aphids. However, much more research needs to be done to prepare for an era of (i) increasingly severe virus epidemics and (ii) increasing difficulties in controlling them, so as to mitigate their detrimental effects on future global food security and plant biodiversity.

Daily temperature extremes play an important role in predicting thermal effects

Organisms in natural environments experience diel temperature fluctuations rather than constant temperatures, including sporadic extreme conditions. Studies based mainly on model organisms have tended to focus on responses to average temperatures or short-term heat stress, which overlooks the potential impact of daily fluctuations including stressful daytime periods and milder nighttime periods. Here we focus on daily maximum temperatures, while holding nighttime temperatures constant, to specifically investigate high temperature effects on demographic parameters and fitness in the English grain aphid, Sitobion avenae (Fabricius). We then compared the observed effects of different daily maximum temperatures with predictions from constant temperature-performance expectations. Moderate daily maximum temperatures depressed aphid performance while extreme conditions had dramatic effects even when mean temperatures were below the critical maximum. Predictions based on daily average temperature underestimated negative effects of temperature on performance by ignoring daily maximum temperature, while predictions based on daytime maximum temperatures overestimated detrimental impacts by ignoring recovery under mild nighttime temperatures. Our findings suggest that daily maximum temperature will play an important role in regulating natural population dynamics and should be considered in predictions. These findings have implications for natural population dynamics particularly when considering the expected increase in extreme temperature events under climate change.

Upsetting the order: how climate and atmospheric change affects herbivore-enemy interactions

Gaining a better understanding of climate and atmospheric change effects on species interactions is one of the great challenges facing modern ecology. Here, we review the literature concerning the responses of insect herbivores and their natural enemies to atmospheric and climate change, focusing specifically on elevated concentrations of atmospheric CO₂ and air temperatures. We recommend that future work on the responses of systems to climate change incorporates as far as possible the trophic complexity inherent in ecosystems, and where feasible, considers the effects of interrelated climate factors in tandem. Such studies will produce more realistic insights into how species interactions may respond under future climates.

Multiple ant species tending lac insect Kerria yunnanensis (Hemiptera: Kerriidae) provide asymmetric protection against parasitoids

This study investigated the effects of ant attendance on the parasitoid community and parasitism of lac insect Kerria yunnanensis aggregations in Yunnan province, China. We manipulated ant attendance to establish three treatments: (1) ant exclusion; (2) low ant attendance by several ant species; and (3) high ant attendance by Crematogaster macaoensis. Five parasitoid species were collected, with two species contributing 82.7 and 13.2% of total abundance respectively. Total parasitoid abundance was lowest in the February sample when K. yunnanensis was in its younger life stage, being significantly lower in the ant exclusion treatment. In April, all three treatments had significantly different parasitoid abundances, being highest in the ant exclusion treatment and the lowest in the high ant attendance treatment. When ants were present, there were strong negative relationships between total parasitoid abundance and ant abundance, with the relationships being dependent upon the ant species composition and abundance. The patterns of total parasitoid abundance were driven by the two most abundant parasitoid species. Parasitoid species richness did not differ among treatments or between sample times, however, multivariate analysis confirmed that overall parasitoid community structure differed significantly among treatments and between sample times, with the high ant attendance treatment differing most from the other two treatments. Interestingly the absence of ants did not result in increased parasitism from four of the five parasitoids. Ants in lac insect farming systems have a clear role for agricultural pest management. A full understanding of the asymmetric abilities of ants to influence parasitoid communities, and affect parasitism of hosts will require further experimental manipulation to assess the relative roles of 1) the abundance of each individual ant species on parasitoid access to hosts, 2) competition among parasitoids, and 3) the interaction between the first two factors.

Constancy in an inconstant world: moving beyond constant temperatures in the study of reptilian incubation

Variable environmental conditions can alter the phenotype of offspring, particularly in ectothermic species such as reptiles. Despite this, the majority of studies on development in reptiles have been carried out under constant conditions in the laboratory, raising the question of just how applicable those investigations are to natural conditions? Here, we first review what we have learned from these constant-temperature studies. Second, we examine the importance of temperature fluctuations for development in reptiles and highlight the outcomes of studies conducted under fluctuating conditions. Next, we report our findings from a new study that examines how the frequency of fluctuations in temperature experienced during development affects phenotype. Finally, we suggest some areas in need of additional research so that we can better understand the complex interactions of temperature and physiology, particularly in species with temperature-dependent sex determination. For questions aimed at understanding the complex effects of the environment on phenotype, we must move toward studies that better capture environmental variation. By taking such an approach, it may be possible to predict more accurately how these thermally sensitive organisms will respond to environmental perturbations, including climatic change.